Cell voltage detecting device for combination battery

ABSTRACT

A device for detecting a voltage of an individual unit cell included in a combination battery consisting of a number of unit cells connected in series. The unit cells are divided into several groups, and a voltage divider circuit having a cell-side resistor and a reference-side resistor is connected to each unit cell. A divided potential of each unit cell is supplied to a cell voltage detector from each voltage divider circuit. Also, a reference potential that is common to a cell group is supplied to the detector from a junction connecting two neighboring cell groups. The cell voltage detector determines the voltage of each unit cell, group by group, based on a difference between the divided potential and the reference potential. Since the reference potential is common to all the voltage divider circuits in one cell group, the voltage divider circuits can be simplified. To further simplify the circuits, the reference-side resistors for the first cell group are commonly used as the reference-side resistors for other cell groups.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims benefit of priority ofJapanese Patent Applications No. Hei-10-298194 filed on Oct. 20, 1998,No. Hei-10-298205 filed on Oct. 20, 1998, No. Hei-10-305969 filed onOct. 27, 1998, and No. Hei-10-373281 filed on Dec. 28, 1998, thecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for detecting individual unitcell voltage in a combination battery that includes a plurality of unitcells connected in series.

2. Description of Related Art

JP-A-5-64377 discloses a device for detecting unit cell voltage in acombination battery that includes a plurality of unit cells. A voltageof an individual unit cell in the combination battery is detected by adifferential-type voltage detecting circuit which is connected to eachunit cell. Since the combination battery usually includes a large numberof unit cells and the same number of the detector circuits is necessaryin this device, the voltage detector is large in size, consumes highpower. Moreover, its manufacturing cost is high, and its reliability isnot sufficiently high.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentionedproblem, and an object of the present invention is to provide animproved device for detecting unit cell voltage in a combinationbattery, which is simple in the circuit structure and has high detectionaccuracy and high reliability.

The combination battery consists of a number of unit cells connected inseries, and the unit cells are divided into several cell groups. Onecell group includes, for example, five unit cells, and four cell groupsare connected in series, constituting a combination battery. The unitcell voltage detecting device is composed of voltage divider circuits,each connected to a plus terminal of each unit cell, and a cell voltagedetector connected to the voltage divider circuits. Each voltage dividercircuit includes at least a cell-side resistor connected to the unitcell and a reference-side resistor connected to a reference potentialline. The cell-side resistor and the reference-side resistor areconnected in series in the voltage divider circuit. The referencepotential line is connected to a common terminal between the cellgroups, for example, between the first cell group and the second cellgroup.

A divided potential of each unit cell is supplied to a high potentialside of the cell voltage detector from a junction between the cell-sideresistor and the reference-side resistor. A reference potential of eachcell group is supplied to a low potential side of the cell voltagedetector from the common terminal that is a junction of a neighboringcouple of cell groups. The unit cell voltage is detected based on thepotential difference between the divided potential and the referencepotential. The reference-side resistor of each voltage divider circuitin the first cell group is commonly used as a reference-side resistor ofa corresponding voltage divider circuit in other cell groups. Forexample, the reference-side resistor of the first unit cell in the firstcell group is common to the reference-side resistor of the first unitcell in the second cell group. The reference potential line includes aswitch for closing and opening the reference potential line, so that thecell voltages are detected cell group by cell group. The reference-sideresistors may be replaced with a common reference-side resistor byproviding a switch in each voltage divider circuit.

Since the reference potential is commonly supplied to each voltagedivider circuit in one cell group, and the reference-side resistors inone cell group are common to the other cell groups, the structure of thevoltage divider circuits is simplified, compared with that of aconventional detector using a separate voltage divider circuit for eachunit cell. Since the divided potential is supplied to the cell voltagedetector, the detector can be operated under a lower voltage.

A reverse-current-preventing diode and/or a current-restricting-resistormay be inserted in the reference potential line to prevent and/orrestrict current short-circuiting a cell group when two referencepotential lines are simultaneously closed accidentally or due tomalfunction of switches in the circuit. All the switches in the voltagedivider circuits and in the reference potential lines may be integratedinto a single module to further simplify the circuit structure.

To compensate possible detection errors due to unevenness amongresistance values in the voltage divider circuits or other causes, afunction to adjust the detected cell voltages may be included in thedevice. For example, an offset voltage may be subtracted from thedetected voltage, and further the detected voltage may be adjusted by anadjusting factor representing a ratio between the detected voltage andthe real voltage. Also, a compensation circuit for simulating a voltagedrop across the reverse-current-preventing diode and the switch insertedin the reference potential line may be added, and the simulated voltagedrop may be subtracted from the divided potential supplied to the cellvoltage detector.

Other objects and features of the present invention will become morereadily apparent from a better understanding of the preferredembodiments described below with reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a cell voltage detecting device as afirst embodiment of the present invention;

FIG. 2 is a circuit diagram showing a cell voltage detecting device as asecond embodiment;

FIG. 3 is a circuit diagram showing a cell voltage detecting device as athird embodiment;

FIG. 4 is a circuit diagram showing an example of a cell voltagedetector circuit;

FIG. 5 is a circuit diagram showing an example of a switch used in thecell voltage detecting device;

FIG. 6 is a circuit diagram showing a cell voltage detecting device as amodification of the first embodiment;

FIG. 7 is a circuit diagram showing a cell voltage detecting device as amodification of the second embodiment;

FIG. 8 is a circuit diagram showing a cell voltage detecting device as amodification of the third embodiment;

FIG. 9 is a circuit diagram showing a cell voltage detecting device as afourth embodiment;

FIG. 10 is a circuit diagram showing an example of a switch used in thefourth embodiment;

FIGS. 11 and 12 jointly show a flowchart of a deviation-compensatingprocess performed in the fourth embodiment;

FIG. 13 is a circuit diagram showing a cell voltage detecting device asa first modification of the fourth embodiment;

FIG. 14 is a circuit diagram showing a cell voltage detecting device asa second modification of the fourth embodiment;

FIG. 15 is a circuit diagram showing an example of a switch used in thecell voltage detecting device; and

FIG. 16 is a circuit diagram showing another example of a switch used inthe cell voltage detecting device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

A first embodiment of the present invention will be described withreference to FIG. 1. A combination battery includes four cell groupsconnected in series, each cell group having five unit cells connected inseries. In FIG. 1, only the first cell group 11 and the second cellgroup 12 are shown, and other cell groups having the same structure areomitted from the drawing for simplicity. The first cell group 11consisting of five unit cells, BAT01, BAT02, BAT03, BAT04 and BAT05connected in series, is located at a highest potential position in thecombination battery. The second cell group 12 consisting of five unitcells, BAT06, BAT07, BAT08, BAT09 and BAT10 connected in series, islocated at a second highest potential position in the combinationbattery. Both cell groups 11 and 12 are connected at a common terminalTc.

A voltage divider circuit consisting of a switch SW1, a cell-sideresistor R1 and a reference-side resistor R2, all connected in series,is connected to a plus terminal of the unit cell BAT01 at one endthereof and to a reference potential line 3 at the other end thereof.Similarly, other voltage divider circuits each having the same structureare connected respectively to other unit cells BAT02, BAT03, BAT04 andBAT05 in the first cell group 11. A voltage divider circuit consistingof a switch SW6, a cell-side resistor R11 and a reference-side resistorR2, all connected in series, is connected to a plus terminal of the unitcell BAT06 at one end thereof and to the reference potential line 3 atthe other end thereof. Similarly, other voltage divider circuits eachhaving the same structure are connected respectively to other unit cellsBAT07, BAT08, BAT09 and BAT10 in the second cell group 12. The cell-sideresistors having odd numbers R1, R3 . . . R17, R19 are provided in eachvoltage divider circuit, while reference-side resistors having evennumbers R2, R4, R6, R8 and R10 are common to both cell groups 11 and 12.All the voltage divider circuits constitute a divider circuit group 2.

A cell voltage detector 4 is connected to the voltage divider circuitsand the reference potential line 3. The cell voltage detector 4 includesfive detector circuits, each being connected to respective junctionsbetween the cell-side resistors (R1, R3 . . . R17, R19) andreference-side resistors (R2, R4, R6, R8, R10) to receive dividedpotentials CH0, CH1, CH2, CH3 and CH4 from the respective voltagedivider circuits. The cell voltage detector 4 is also connected to areference potential line 3 to receive a reference potential VSS. Thereference potential line 3 is further connected to the common terminalTc that is a junction between the first cell group 11 and the secondcell group 12 through a switch SW21. Similarly, another referencepotential line 3 is connected between the reference potential terminalVSS and a lower common terminal Tc through a switch SW22. Each cellvoltage detector circuit is constituted, for example, by an A/Dconverter that includes a reference voltage generator circuit,comparators and a digital signal generator circuit. The referencevoltage generator circuit generates various reference voltages that arehigher than the reference potential VSS. The comparators compare thereference voltages with the divided potentials fed from the voltagedivider circuits, and the digital signal generator circuit converts thesignals fed from the comparators into digital signals. Though five A/Dconverters are used in the detector 4 in this embodiment, a single A/Dconverter which sequentially switches input signals may be used.

Each switch SW1-SW10, SW21 and SW22 is composed of a photo-MOStransistor that is driven by a light signal emitted from a lightemitting diode facing the switch, and thereby the switch is operated inan ON and OFF fashion.

To detect the unit cell voltages in the first cell group 11, the cellvoltage detector 4 receives the reference potential VSS from thereference potential line 3 that is connected to the common terminal Tcthrough the closed switch SW21. At this moment, the switch SW22 is keptopen. The switches SW1-SW5 are all closed, while the switches SW6-SW10are all open. The divided potentials CH0, CH1, CH2, CH3 and CH4 are fedto the cell voltage detector 4 from the respective voltage dividercircuits. The A/D converters in the cell voltage detector 4 convert therespective divided potentials into digital signals which are supplied toa controller (not shown) connected to the cell voltage detector 4. Thecontroller determines the voltage of each unit cell BAT01-BAT05 bysequentially subtracting a lower unit cell potential from a higher unitcell potential. The unit cell voltages in the lower cell group 12 aresimilarly detected. That is, switches SW22 and SW6-SW10 are all closed,while the switches SW21 and SW1-SW5 are all open. Other detectionprocesses are the same as those described above. In short, each unitcell voltage is individually detected by detecting a voltage drop acrossthe reference-side resistors R2, R4, R6, R8 and R10.

Since the divided potentials CH0-CH4 supplied from voltage dividercircuits are compared with the common reference potential VSS appearingon the reference potential line 3, the power source voltage imposed oncell voltage detector circuits can be common to all the cell voltagedetector circuits. Further, since the cell voltage signals fed to thecell voltage detector circuits are divided by the voltage dividercircuits, high potential imposition on the cell voltage detectorcircuits is avoided. Therefore, the structure of the detector circuitsis simplified. Moreover, since the reference-side resistors R2, R4, R6,R8 and R10 are common to all the cell groups, the number of resistorsused in the voltage divider circuits can be reduced.

Referring to FIG. 4, an example of the cell voltage detector circuitwill be described. Five circuits identical to circuit shown in FIG. 4are contained in the cell voltage detector 4. The cell voltage detectorcircuit is composed of a differential voltage circuit 41 and an A/Dconverter 42 that converts the output voltage of the differentialvoltage circuit 41 into a digital signal. A constant voltage powersource 43 supplies voltage VH, VL to the circuit.

To detect the unit cell voltages in the first cell group 11, the switchSW21 is closed and the switch SW22 is opened. The differential voltagecircuit 41 receives the reference potential VSS from the referencepotential line 3 which is connected to the common terminal Tc throughthe switch SW21. The voltage VL supplied from the constant voltagesource 43 is shifted to a level lower than the level of VSS, and thevoltage VH is set at a level higher than the divided potential CH0. Theswitches SW1-SW5 for the first cell group 11 are closed while theswitches SW6-SW10 are opened. Thereby, the divided potential CH0 (orCH1-CH4) are fed to the differential voltage circuit 41. The potentialdifference between CH0 and VSS is fed from the differential voltagecircuit 41 to the A/D converter 42 that converts the signal into adigital signal. The converted digital signal is fed to the controller(not shown) connected to the A/D converter 42. The controller determinesthe voltage of the unit cell, for example, BAT01 by subtracting adigital signal corresponding to the divided potential CH1 from a digitalsignal corresponding to the divided potential CH0. The voltages of otherunit cells are similarly determined.

The unit cell voltages in the second cell group 12 are similarlydetected and determined. In this case, the switches SW22 and SW6-SW10are closed, while other switches are opened.

Second Embodiment

A second embodiment of the present invention is shown in FIG. 2. In thisembodiment, resistors R21 and R22 are added in the reference potentiallines 3, respectively. Namely, the resistor R21 is inserted between theVSS terminal and the switch SW21, and the resistor R22 is insertedbetween the VSS terminal and the switch SW22. A resistance ratio betweenthe cell-side resistor and the reference-side resistor in each voltagedivider circuit is properly adjusted. Other structures of the secondembodiment are the same as those of the first embodiment.

If both switches SW21 and SW22 are accidentally closed at the same timefor any reason including malfunction of the switches, short-circuitedcurrent therethrough is restricted by the resistors R21 and R22, therebyimproving safety of the device.

Third Embodiment

A third embodiment of the present invention is shown in FIG. 3. In thisembodiment, compared with the first embodiment shown in FIG. 1,positions of the cell-side resistors 2a (i.e., R1-R9, R11-R19) and theswitches SW1-SW10 in each voltage divider circuit are reversed. All theswitches SW1-SW10, SW21 and SW22 constituted by MOS transistors areintergrated in a single module. The reference-side resistors R2, R4, R6,R8 and R10 used in the first embodiment are replaced with a commonresistor Rc, and only one divided potential terminal HC, in place ofHC0-CH4, is provided in the cell voltage detector 4 that has only onedetector circuit. Other structures are the same as those of the firstembodiment.

In detecting the voltages of unit cells BAT01-BAT05 included in thefirst cell group 11, the switch SW21 is closed to supply the referencepotential VSS to the cell voltage ilk detector 4 in the same manner asin the first embodiment, but the switches SW1-SW5 are sequentiallyclosed to feed the divided voltages to the CH terminal one by one.Similarly, in detecting the voltages of unit cells BAT06-BAT-10 includedin the second cell groups 12, the switch 22 is closed, and the switchesSW6-SW10 are sequentially closed. The resistors R21 and R22 used in thesecond embodiment may be added in this embodiment, too.

Since the input signals from the unit cells are first fed to thecell-side resistors R1-R19 and then to the switches SW1-SW10,electrostatic noise or other noise possibly superimposed with the inputsignals are attenuated by the cell-side resistors, and thereby theswitches SW1-SW10 are protected against those noises. Since the switchesSW1-SW10, SW21 and SW22 are constituted by MOS transistors, thoseswitches can be driven by a lower voltage.

Referring to FIG. 5, an example of the MOS transistor switch will bedescribed. In FIG. 5, the switch SW1 is shown as an example. The switchSW1 operates under a control voltage Vgs that is a difference between agate potential Vg and a source potential Vs. When the switch SW1 isopen, the source potential Vs is equal to the reference potential VSS ofline 3, and therefore the switch SW1 can be operated under the low Vgs.When the switch SW1 is closed, only the divided potential is imposed onthe switch SW1 as the source potential Vs, and therefore the switch SW1can be driven under the low Vgs.

Referring to FIG. 3 again, the switches SW1-SW10, SW21 and SW22 are allintegrated into a single module. One end of switches SW1-SW10 iscommonly connected to a terminal 51, and one end of switches SW21 andSW22 is commonly connected to another terminal 52. Therefore, thecircuit pattern of the integrated switches can be simplified. Moreover,it is also possible to integrate all the switches in a single chipincluding the connecting circuit. The positions of the cellsideresistors and switches in the voltage divider circuits of the firstembodiment may be reversed as in the third embodiment, and thoseswitches may be integrated in a single chip.

Referring to FIG. 6, a modification of the first embodiment will bedescribed. In this modification, the switches SW1-SW10 in the voltagedivider circuits are positioned between the cell-side resistors R1-R9,R11-R19 and the reference-side resistors R2-R10. Diodes D1 and D2 areadded in the reference potential line 3 to intercept reverse current.Other structures are the same as those of the first embodiment. Sincethe diodes D1 and D2 are added in the reference potential line 3, whenboth switches SW21 and SW22 are accidentally closed for any reasonincluding malfunction of those switches, current short-circuiting theunit cells included in the second cell group 12 is prevented fromflowing through the reference potential line 3.

When the unit cell voltages in the first cell group 11 are beingdetected, a voltage drop across the diode D1 and the switch SW21 isincluded in the detected potential. Therefore, it is preferable tosubtract the voltage drop from the detected potential difference betweenCH4 and VSS to determine the cell voltage of the unit cell BAT05 with ahigher accuracy. The voltage drop across the diode D1 and the switchSW21 may be calculated in a program installed in the cell voltagedetector 4 based on the voltages of unit cells BAT01-BAT05 andtemperature of the diode D1 and the switch SW21. The same as aboveapplies to detection of the cell voltages in the second cell group 12.The voltage drop across the diode D2 and the switch SW22 is similarlycalculated.

Referring to FIG. 7, a modification of the second embodiment will bedescribed. In this modification, the switches SW1-SW10 are positionedbetween the cell-side resistors R1-R19 and the reference-side resistorsR2-R10, and diodes D1 and D2 are added in the reference potential lines3, respectively. Other structures are the same as those of the secondembodiment. Since the diodes D1 and D2 are added, the currentshort-circuiting the unit cells in the second cell group 12 is preventedfrom flowing through the reference line 3, even if both switches SW21and SW22 are simultaneously closed for any reason.

Referring to FIG. 8, a modification of the third embodiment will bedescribed. In this modification, diodes D1 and D2 are added in thereference potential lines 3, respectively. Other structures are the sameas those of the third embodiment, and the device operates in the samemanner. The diodes D1 and D2 prevent the current short-circuiting thecells in the second cell group 12, even if both switches SW21 and SW22are closed simultaneously.

Fourth Embodiment

A fourth embodiment of the present invention will be described withreference to FIGS. 9-12. The circuit structure of this embodiment issimilar to that of the first embodiment, except for the diodes D1, D2added in the reference potential lines 3 and a compensation circuit 5added for canceling a voltage drop across the diode D1 and the switchSW21 (or a voltage drop across the diode D2 and the switch SW22). Onlythe structure and function different from the first embodiment will bedescribed below.

The diodes D1, D2 prevent the current short-circuiting the unit cells inthe second cell group 12 upon simultaneous closure of both switchesSW21, SW22, in the same manner as in other similar embodiments describedabove.

As shown in FIG. 9, the compensation circuit 5 is composed of a dummyresistor Rd, a dummy switch SWd and a dummy diode Dd, all connected inseries. One end of the compensation circuit 5 is connected to a junctionbetween the switch SW21 and the diode D1, and the other end thereof isconnected to the reference potential line 3. A junction between thedummy resistor Rd and the dummy switch SWd is connected to a CH5terminal of the cell voltage detector 4. The resistance of the dummyresistor Rd is selected, so that the same amount of current, as thecurrent that flows through the diode D1 upon closure of switches SW1-SW5and SW21 to detect the cell voltages in the first cell group 11 (whichis equal to the current that flows through the diode D2 upon closure ofswitches SW6-SW10 and SW22 to detect the cell voltages in the secondcell group 12), flows through the compensation circuit 5 upon closure ofthe dummy switch SWd. The dummy switch SWd is made in the same processas the switches SW21 and SW22, so that its characteristics are the sameas those of the switches SW21 and SW22. Also, the dummy diode Dd issimulated to the diodes D1 and D2.

The compensation circuit 5 functions as follows. The switches SW1-SW10are all opened, and both switches SW21, SW22 and the dummy switch SWdare closed. A current that flows through the compensation circuit 5under these conditions causes a voltage drop Vd across the dummy switchSWd and the dummy diode Dd. The voltage drop Vd is substantially thesame as the voltage drop appearing across the diode D1 and the switchSW21 when the cell voltages in the first cell group 11 are detected (orthe voltage drop appearing across the diode D2 and switch SW22 when thecell voltages in the second cell group 12 are detected). The signal ofthe voltage drop Vd is fed to the cell voltage detector 4 as a potentialCH5 which is processed by the cell voltage detector 4 and fed to thecontroller in the same manner as other divided potentials CH0-CH4,because an additional detector circuit that is the same as otherdetector circuits for processing the potentials CH0-CH4 is included inthe cell voltage detector 4. The voltage of the unit cell BAT05 that islocated at the bottom of the first cell group 11 is correctly detectedby subtracting the potential CH5 from the potential CH4. Similarly, thevoltage of the unit cell BAT10 located at the bottom of the second cellgroup 12 is correctly detected.

The voltages of the unit cells other than BAT05 (located at the bottomof the first cell group 11) and BAT09 (located at the bottom of thesecond cell group 12) a re determined by subtracting a potential at anext lower cell terminal from a potential at each cell terminal afterthe divided potentials are converted into digital signals. Thus, thevoltage drop across the diode D1 and the switch SW21 (or the voltagedrop across the diode D2 and the switch SW22) is canceled. It is alsopossible to determined a cell voltage by directly converting a potentialdifference between a pair of neighboring cells into a digital signal inthe A/D converter.

Referring to FIG. 10, one example of the switch SW1-SW10 that isconstituted by a MOSFET will be described. In FIG. 10, the switch SW1 isshown as an example. The switching operation is performed under acontrol voltage Vag that is a difference between a gate voltage Vg and asource voltage Vs of the MOSFET. Because the source voltage Vs is equalto the reference potential VSS when the switch SW1 is open, the switchSW1 can be operated under a low control voltage Vgs. The source voltageVs is the divided voltage when the switch SW1 is closed, and thereforethe switch SW1 can be sufficiently operated under a low control voltageVgs. If the switch SW1 is positioned between the plus terminal of theunit cell BAT01 and the resistor R1, the source voltage Vs becomessubstantially equal to the plus terminal voltage when the switch SW1 isclosed, and the gate voltage Vg has to be higher than the plus terminalvoltage. Therefore, in this case, a larger MOSFET having a highervoltage durability has to be used.

Referring to FIGS. 11 and 12, a process for compensating deviation of adetected cell voltage from a real cell voltage due to resistanceunevenness among the resistors constituting the voltage divider circuitswill be described. 20 It is, of course, preferable to make theresistance values of all the cell-side resistors R1-R19 equal, and tomake the resistance value of all the reference-side resistors R2-R10equal. It is not realistic, however, to eliminate all the unevennessamong those resistors. The following process is to 25 compensatedetected cell voltage deviation caused by the unevenness of theresistors. A routine for the deviation-compensating process is shown ina flowchart of FIGS. 11 and

At step S1: First, the combination battery 1 is detached from the cellvoltage detecting device, and all the cell-side ends of the resistorsR1, R3, R5, R7 and R9 are short-circuited and connected to the commonterminal Tc. The switches SW1-SW5 and SW21 are closed, and otherswitches are all opened. Then, output voltages from five cell voltagedetector circuits are respectively detected and memorized as offset cellvoltages ΔVn (n=1-5). Similarly, all the cell-side ends of the resistorsR11, R13, R15, R17 and R19 are short-circuited and connected to thelower common terminal Tc. The switches SW6-SW10 and SW22 are closed, andother switches are all opened. Then, output voltages from five cellvoltage detector circuits are respectively detected and memorized asoffset cell voltages ΔVn (n=6-10). Thus, ten offset voltages ΔVn(n=1-10) are detected and memorized. Each offset voltage ΔVn is equal tothe output voltage from the cell voltage detector circuit when no actualcell voltage is fed thereto. Then, switches SW21, SW22 and SWd areclosed and all other switches are opened, and the output voltage fromthe detector circuit is detected and memorized as an offset voltage dropΔVd. The offset voltage drop ΔVd is equal to the voltage drop across thedummy switch SWd and the dummy diode Dd when no actual voltage is fed tothe compensation circuit 5.

At step S2: The combination battery 1 and the cell voltage detectingdevice are connected in a normal way as shown in FIG. 9. SwitchesSW1-SW5 and SW21 are closed and all other swithches are opened. Then,the output voltages from five cell voltage detector circuits arerespectively detected and memorized as output cell voltages Vn (n=1-5).The voltages of the unit cells BAT01-BAT05 are actually measured andmemorized as real cell voltages VRn (n=1-5). Similarly, SwitchesSW6-SW10 and SW22 are closed and all other switches are opened. Then,the output voltages from five cell voltage detector circuits arerespectively detected and memorized as output cell voltages Vn (n=6-10).The voltages of the unit cells BAT06-BAT10 are actually measured andmemorized as real cell voltages VRn (n=6-10). Thus, all the output cellvoltages Vn (n=1-10) and all the real cell voltages VRn (n=1-10) aredetected and memorized. Then, the switches SW21, SW22 and SWd are closedand all other switches are opened, and the output form the detectorcircuit is detected and memorized as a voltage drop Vd. The voltage dropacross the dummy switch SWd and the dummy diode Dd is actually measuredand memorized as a real voltage drop VRd.

At step S3: The offset cell voltage drop ΔVn is subtracted from theoutput cell voltage Vn to obtain an offset-compensated cell voltage(Vn-ΔVn) for each unit cell. Then, the offset-compensated cell voltage(vn-Δvn) is divided by the corresponding real cell voltage VRn to obtainan adjusting factor Kn (n=1-10). Similarly, the offset voltage drop ΔVdis subtracted from the voltage drop Vd to obtain an offset-compensatedvoltage drop (Vd-ΔVd). The offset-compensated voltage drop (Vd-ΔVd) isdivided by the real voltage drop VRd to obtain an adjusting factor R.

At step S4: The offset-compensated cell voltage (Vn-ΔVn) for each unitcell calculated at step S3 is installed here again.

At step S5: The voltage of each cell BAT01-BAT10 is finally determinedas VDn (n=1-10) by dividing the offset-compensated cell voltage (Vn-ΔVn)by the corresponding adjusting factor Kn.

The process performed at steps S1-S5 is summarized as follows. All thevoltage divider circuits and the cell voltage detector 4 are assumed asan unknown black box, and the output cell voltage Vn is expressed by theformula: Vn=Kn·VRn+ΔVn. The offset cell voltage ΔVn is detected as theoutput voltage when no cell voltage is supplied to the detector. Thereal cell voltage VRn is actually measured, and the output cell voltageVn is detected by the detector. Then, the adjusting factor Kn iscalculated from the formula: Kn=(Vn-ΔVn)/VRn. Finally, the cell voltageVDn is determined by dividing the offset-compensated cell voltage by theadjusting factor: VDn=(Vn-ΔVn)/Kn=VRn.

At step S6: The voltages of unit cells other than BAT05 (the cell at thebottom of the first cell group 11) and BAT10 (the cell at the bottom ofthe second cell group 12) are finalized by subtracting the voltage of alower level cell in the same manner as described above.

At step S7: An offset-compensated voltage drop is calculated bysubtracting the offset voltage drop ΔVd memorized at step Si from thevoltage drop Vd detected at step S2. Namely, the offset compensatedvoltage drop=(Vd-ΔVd).

At step S8: The final voltage drop VDd across the dummy switch SWd andthe dummy diode Dd is determined by dividing the offset-compensatedvoltage drop by the adjusting factor R calculated at step S3. Namely,VDd=(Vd-ΔVd)/R.

At step S9: The voltages of the unit cells BAT05 and BAT10 are finallydetermined by subtracting VDd from the VDn.

Though the process described above is easily performed by amicrocomputer, the process may be modified or simplified.

A modification of the fourth embodiment is shown in FIG. 13. In thismodification, the compensation circuit 5 of the fourth embodiment issimplified. A compensation circuit 30 is connected between the minusterminal of the diode D1 and the CH5 potential terminal, and the plusterminal of the diode D1 is connected to the reference potential line 3having the reference potential VSS. The voltage drop across the diode D1is detected by the detector 4. The cell voltage of BAT05 is determinedby subtracting the voltage drop across the diode D1 from the dividedpotential of BAT05. Another compensation circuit 31 is connected betweenthe minus terminal of the diode D2 and a potential terminal CH6, and theplus terminal of the diode D2 is connected to the reference potentialVSS. The voltage drop across the diode D2 is similarly detected by thedetector 4. The cell voltage of BAT10 is determined by subtracting thevoltage drop across the diode D2 from the divided potential of BAT10.The deviation compensating process used in the fourth embodiment isapplicable to this modification, too.

The voltage drop across the diode D1 and the switch SW21 (or D2 andSW22) may also be calculated in the following manner. The voltage dropat a room temperature is memorized in a microcomputer, and it isadjusted according to an operating temperature of the device. Theoperating temperature is detected by a temperature sensor or the like.The temperature-dependent characteristic of the voltage drop across thediode D1 and the switch SW21 is pre-installed in the microcomputer as amap. The voltage drop across the diode D2 and the switch SW22 issimilarly calculated.

Another modification of the fourth embodiment is shown in FIG. 14. Inthis modification, one end of the compensation circuit 5 is directlyconnected to the common terminal Tc. Since the switch SW21 is notincluded in a circuit from the common terminal Tc to the referencepotential terminal CH5, the voltage drop Vd detected by the compensationcircuit 5 better simulates the actual voltage drop across D1 and SW21(or D2 and SW22).

Referring to FIG. 15, one exemplary structure of the switch SW1-SW10will be explained, taking the switch SW10 as an example. The switch SW10is composed of a MOS transistor 100 and a reverse-current-preventingdiode 101. A parasitic diode of the MOS transistor 100 is shown as Dx.When the switch SW10 is structured in this form, no short-circuitingcurrent flows through the switches SW5 and SW10 if both switches aresimultaneously closed by accident or for any other reason.

Referring to FIG. 16, another exemplary structure of any of the switchesSW1-SW10 , SW21 and SW22 will be explained, taking the switch SW10 as anexample. The switch SW10 is composed of a pair of MOS transistors 100and 102 which are connected so that both parasitic diodes Dx areconnected in opposite directions to each other. In this switchstructure, no short-circuiting current flows through the switches SW5and SW10 even if both swathes are simultaneously closed. The same istrue, of course, for other pairs of switches, SW1 and Sw6, SW2 and SW7,and so forth. The reverse-current-preventing diode 101 used in theswitch shown in FIG. 15 may be added for further safety.

While the present invention has been shown and described with referenceto the foregoing preferred embodiments, it will be apparent to thoseskilled in the art that changes in form and detail may be made thereinwithout departing from the scope of the invention as defined in theappended claims.

What is claimed is:
 1. A unit cell voltage detecting device for acombination battery consisting of a plurality of unit cells connected inseries, the plurality of unit cells being divided into several cellgroups, the unit cell voltage detecting device comprising:a plurality ofvoltage divider circuits each connected to each unit cell, the voltagedivider circuit having at least a cell-side resistor connected to a plusterminal of the unit cell and a reference-side resistor connected inseries to the cell-side resistor; a plurality of reference potentiallines, each connecting a common terminal representing a lowest potentialof each cell group to the reference-side resistors of that cell group;and a cell voltage detector for detecting a cell voltage of each unitcell, a high potential side of the cell voltage detector being connectedto a junction of the cell-side resistor and the reference-side resistorfor receiving a divided potential therefrom, a low potential side of thecell voltage detector being connected to the reference potential linefor receiving a reference potential therefrom, wherein:each unit cellvoltage is detected based on a potential difference between the dividedpotential and the reference potential.
 2. The unit cell voltagedetecting device as in claim 1, wherein:a first switch for opening andclosing the reference potential line is disposed in the referencepotential line; a second switch for opening and closing the voltagedivider circuit is disposed in the voltage divider circuit; and thereference-side resistor in the voltage divider circuit for a unit cellin a first cell group having a highest potential is commonly used as thereference-side resistor for a corresponding unit cell in a second cellgroup and other cell groups.
 3. The unit cell voltage detecting deviceas in claim 2, wherein:a reverse-current-preventing diode is disposed inthe reference potential line, so that current short-circuiting thesecond cell group is prevented from flowing through the referencepotential line when first switches for the first and second cell groupsare simultaneously closed.
 4. The unit cell voltage detecting device asin claim 3, wherein:a resistor is further connected in series to thereverse-current-preventing diode in the reference potential line.
 5. Theunit cell voltage detecting device as in claim 2, wherein:a resistor isdisposed in the reference potential line, so that an amount of currentshort-circuiting the second cell group is suppressed when first switchesfor the first and second cell groups are simultaneously closed.
 6. Theunit cell voltage detecting device as in claim 2, wherein:a singleresistor is commonly used as the reference-side resistor for all thevoltage divider circuits.
 7. The unit cell voltage detecting device asin claim 1, wherein:a second switch for closing and opening the voltagedivider circuit is disposed in the voltage divider circuit; and a singleresistor is commonly used as the reference-side resistor for all thevoltage divider circuits in each cell group.
 8. The unit cell voltagedetecting device as in claim 1, wherein:a first switch for closing andopening the reference potential line is disposed in the referencepotential line; a second switch for closing and opening the voltagedivider circuit is disposed between the cell-side resistor and thereference-side resistor in the voltage divider circuit; and the firstand second switches for all the cell groups are integrated in a singlecircuit module.
 9. The unit cell voltage detecting device as in claim 1,further including:memory means for storing information adjusting adifference between a real cell voltage and a detected cell voltage andmeans for adjusting the detected cell voltage based on the storedinformation.
 10. The unit cell voltage detecting device as in claim 9,wherein:the memory means stores a ratio of the detected cell voltage tothe real cell voltage; and the adjusting means divides the detectedvoltage by the ratio to obtain the real cell voltage.
 11. The unit cellvoltage detecting device as in claim 10, wherein:the memory meansmemorizes an offset cell voltage that is an output voltage of the cellvoltage detector when the combination battery is detached from thedetecting device; and the adjusting means calculates the detected cellvoltage by subtracting the offset cell voltage from the output voltageof the cell voltage detector.
 12. The unit cell voltage detecting deviceas in claim 9, wherein:the memory means memorizes an offset cell voltagethat is an output voltage of the cell voltage detector when thecombination battery is detached from the detecting device; and theadjusting means calculates the detected cell voltage by subtracting theoffset cell voltage from the output voltage of the cell voltagedetector.
 13. The unit cell voltage detecting device as in claim 1,wherein:a reverse-current-preventing diode and a first switch forclosing and opening the reference potential line connected in series aredisposed in the reference potential line; a second switch for closingand opening the voltage divider circuit is disposed in each voltagedivider circuit; a compensation circuit for simulating a voltage dropacross at least the reverse-current-preventing diode is connected to thereference potential line; and the cell voltage detector subtracts thesimulated voltage drop from the divided potential received from thevoltage divider circuit in a process to determine the cell voltage. 14.The unit cell voltage detecting device as in claim 13, wherein:thevoltages of the unit cells located at a lowest potential position ineach cell group are determined based on the divided potential from whichthe simulated voltage drop is subtracted; and the voltages of unit cellsother than the unit cells located at the lowest potential position ineach cell group are determined based on a divided potential differencebetween the neighboring cells.
 15. The unit cell voltage detectingdevice as in claim 13, wherein:the compensation circuit simulates avoltage drop across the first switch and the reverse-current-preventingdiode disposed in the reference potential line.
 16. The unit cellvoltage detecting device as in claim 1, wherein:areverse-current-preventing diode and a first switch for closing andopening the reference potential line connected in series are disposed inthe reference potential line; a second switch for closing and openingthe voltage divider circuit is disposed in each voltage divider circuit;and the cell voltage detector includes a memory for prestoring a voltagedrop across the reverse-current-preventing diode and the first switch,so that the prestored voltage drop is subtracted from the dividedpotential supplied to the cell voltage detector from the voltage dividercircuit in determining the unit cell voltage.
 17. The unit cell voltagedetecting device as in claim 16, wherein:the prestored voltage drop inthe memory is adjusted based on a detected operating temperature of thecell voltage detecting device.
 18. The unit cell voltage detectingdevice as in claim 1, wherein:a reverse-current-preventing diode and afirst switch for closing and opening the reference potential lineconnected in series are disposed in the reference potential line; asecond switch for closing and opening the voltage divider circuit isdisposed in each voltage divider circuit; and the cell voltage detectorincludes means for detecting a voltage drop across thereverse-current-preventing diode and the first switch disposed in thereference potential line, so that the voltage drop is subtracted fromthe divided potential in determining the unit cell voltage.